The invention relates to a method for monitoring the cycle of weft insertion into a weaving machine with a signal generating sensor activated by yarn deflection and connected to an evaluation unit.
In known weaving machines, the cycle of the weft insertion is determined by a previously set program and is monitored by yarn feelers of mechanical, capacitive, tribo-electrical or opto-electrical types. In order to assure a reliable response of said sensors to a yarn breakage the sensors have to react relatively slowly, i.e. by a response time with a magnitude 10 ms or more. From insertion to insertion in this way the cycle of the yarn movement during the weft insertion can be determined only vaguely by measuring the response points in time of different sensors provided along the yarn path. A continuous measuring and monitoring of the yarn movement during the weft insertion herewith is excluded. Also an optimization of the cycle of the weft insertion, e.g. by a target control of the air nozzles of the air jet weaving machine, is impossible in this case. Furthermore, it is difficult to detect problems of a weft insertion early enough. A reliable stop of the weaving machine in case of insertion disturbances, however, is a prerequisite so as to avoid fabric faults. For these reasons the existing sensors are frequently adjusted so sensitively that they stop the weaving machine even in a doubtful case. This leads to an increased demand for interferences by an operator.
According to the method as known from EP 0 117 571 A for monitoring the feeding state of a yarn during a weft insertion into a weaving machine, a tuning fork is actuated by the weft yarn which tuning fork during movement of the weft yarn transmits oscillations to a sensor provided e.g. with piezo-electric material. The movement of the weft yarn is detected and monitored in order to derive a signal exclusively indicating the running movement of the weft yarn. By means of a diagnosis, it is concluded that a yarn breakage has occurred from a yarn stop which occurs at a point in time which would not normally be expected. The yarn force resulting from the tension in weft yarn is not measured. Irrespective of the momentary weft yarn tension, the sensor does not generate a signal when the weft yarn has stopped.
According to the weft yarn monitoring method known from U.S. Pat. No. 3,688,958 A, the sensor provided only generates a signal if the weft yarn is running and even first if the weft yarn has reached a predetermined running speed. The frequency of yarn irregularities rubbing at the sensor during the yarn run is measured, but not the yarn force.
The pulling force in the weft yarn occasionally is measured for scientific purposes in an experimental manner. Sensors used for this purpose employ strain measuring strips forming mechanical-electrical transducers. The materials used limit the sensitivity, the capability to withstand overloads, and the limit frequency such that only carefully prepared laboratory measurements can be carried out for single insertion cycles and only on particularly robust yarns which can stand the additional load at the deflection points of the sensors. For an industrial production the measuring method cannot be used, and also cannot be used because of the limited life duration, the complicated handling, and the high costs of those experimental apparatus.
It is an object of the present invention to measure the yarn force during a weft insertion with a reasonably priced, robust, accurate and quick-reacting sensor, and to optimize and more reliably monitor the cycle of the weft insertion. The sensor is based on the principle of yarn deflection. The deflection angle amounts to less than 45xc2x0, preferably less than 30xc2x0. The limit frequency of the sensor is set above 1 kHz, and preferably above 5 kHz. The sensor preferably is realized with a piezo-resistive or piezoelectric crystal. For the piezo-resistive measuring principle e.g. a force sensor type PK 8870 made by the Honeywell Company is used. The sensor is employed in co-action with a direct voltage amplifier having a limit frequency of at least 1 kHz, and preferably more than 5 kHz. For carrying out the piezo-electric measuring principle e.g. a force sensor of the production program of the Kistler Company is employed, in co-action with a charge amplifier. In this case, a quasi static output signal is generated by respectively resetting the amplifier in the forceless phase of the insertion cycle. Details of the piezo-electrical measuring method are described in detail in the sales documentation of the Kistler Company.